As IC device geometries shrink, development of low-k inter-metal dielectrics (IMDs) becomes more important. Current production processes use dense films that have a k-value between 3 and 4. These films are typically deposited using Plasma Enhanced Chemical Vapor Deposition (PECVD) processes and are typically made from either fluoro-silicate glass (FSG) or organo-silicate glass (OSG). Devices in current production range between 90 nm and 120 nm in gate width. The Semiconductor Industry roadmap calls for further shrinking these device geometries to 65 nm, then 45 nm and beyond over the remainder of this decade.
As device geometries shrink further, there is a need for IMD films with k-values under 2.7. Successfully developing a film of such low capacitance requires including porosity in the film. To this end, ultra-low-k (ULK) IMD films of porous OSG have been developed. These ULK films are deposited using PECVD techniques, wherein an OSG backbone and a pore generator (porogen) are co-deposited on a semiconductor wafer. Various techniques such as thermal, ultraviolet (UV) and electron beam curing are then used to drive the porogen out of the composite film leaving behind a porous OSG matrix. The resulting porous film exhibits k-values ranging from 2.0 to 2.5 due to the presence of pores containing air, which by definition has a k of 1.0.
However, the inclusion of pores in these films renders them softer and mechanically weaker than dense OSG films. Mechanical strength and hardness are necessary for the film to survive subsequent processing steps from chemical mechanical polishing to the various wire-bonding steps during chip packaging. Therefore, to compensate for the mechanical weakness introduced by the pores in these ULK films, the OSG backbone needs to be strengthened. Further processing of these wafers using UV radiation and electron beams increases cross-linking, which strengthens the film. Thermal curing has no further effect on the mechanical properties of the film after the porogen has been driven out, and therefore cannot be used to harden or strengthen the film.
However, the process of curing ULK films on semiconductor wafers is time consuming. Typical cure times may exceed 5 minutes. Two ways to decrease cure time are by increasing wafer temperature and increasing UV intensity. However, any increase in wafer temperature is limited due to the effect of high temperature on the underlying layers, specifically copper. Higher temperatures cause copper agglomeration, a phenomenon also referred to as “copper hillock” formation. The potential for increasing UV intensity is limited by the commercial availability of UV lamps themselves. The most intense source of UV radiation continues to be traditional mercury-vapor lamps. However, these lamps require extensive air-cooling, provision for which limits the number of lamps that can be packaged above a standard semiconductor wafer.
In addition, curing with commercially available mercury-vapor UV lamps often results in areas of non-uniformity on the wafer because the tubular geometry of these lamps is not optimized for uniform illumination of a wafer. Further, there are significant variations in UV output from lamp to lamp.
Mercury-vapor lamps exhibit another significant shortcoming, namely that in order to generate said vapor, the lamps must operate at significantly higher temperatures than is desired for processing ULK films. Typical commercial lamp systems cause the lamp envelope to reach temperatures between 800° and 900° C. This requires sophisticated wafer-temperature control schemes wherein the infra-red (IR) energy that is radiatively coupled to the wafer from the lamp envelopes needs to be removed while maintaining the wafer at the desired operating temperature, typically in the range between 250° and 400° C.
Therefore, there is a need for inventions that improve uniformity of curing and wafer throughput given the limitations in both the wafer temperature and UV intensity from available lamps.